The present invention is directed to providing logic level signal indication of whether a first signal in a phase-locked loop is locked in phase and frequency to a second signal provided to the loop.
In prior art systems, a phase comparator is coupled to the phaselocked loop for providing a first correction signal when the first and second signals are out of phase in one direction and a second correction signal when the first and second signals are out of phase in the opposite direction; and a lock detector is coupled to the phase comparator for providing a first digital logic signal having a first state whenever either the first correction signal or the second correction signal is provided from the phase comparator, and a second state when neither correction signal is provided. The second state thereby indicates that the first signal is locked in phase to the second signal.
The problem with these prior art systems is that every single correction signal pulse produces an equal duration pulse at the lock detector output. Since there are always correction signal pulses occuring in a phase-locked loop system, the lock detector output is constantly pulsing. This output can be integrated, but then an analog signal is produced (as opposed to a digital logic level signal) for indicating whether the loop is in or out of lock. In addition, with an analog signal the pulse width that can be discriminated as indicating an "out of lock" condition is excessively wide.